Southern Utah in August is scorching hot! Remember this is a desert and the national parks are for the most part a portion of the desert landscape. Temperatures in Moab and near Zion regularly go into the range between 100 and 110 or higher. Please don't think I'm dissin' the area, it's beautiful and things you will see no where else. The timing is something you might consider. As was mentioned before May and June will be the melt months and the highest, fastest water there. Regardless of your timing, it is a wonderful part of our country.